As benchmarks for judging the catalytic power of sulfate monoesterases, we sought to determine the rates of spontaneous hydrolysis of unactivated alkyl sulfate monoesters by S-O bond cleavage. Neopentyl sulfate proved to be unsuitable for this purpose, since it was found to undergo hydrolysis by a C-O bond cleaving mechanism with rearrangement of its carbon skeleton. Instead, we examined the temperature dependence of the spontaneous hydrolyses of aryl sulfate monoesters, which proceed by S-O cleavage. Extrapolation of a Bronsted plot [log(k(25)(N)) = (-1.81 +/- 0.09) pK(a)(LG) + (3.6 +/- 0.7)] based on the rate constants at 25 degrees C for hydrolysis of a series of sulfate monoesters to a pK(a)(LG) value of 16.1, typical of an aliphatic alcohol, yields k(25)(N) = 3 x 10(-26) s(-1) Comparison of that value with established k(cat) values of bacterial sulfatases indicates that these enzymes produce rate enhancements (k(cat)/k(uncat)) fo up to 2 x 10(26)-fold for the hydrolysis of sulfate monoesters. These rate enhancements surpass by several orders of magnitude the similar to 10(21)-fold rate enhancements that are generated by phosphohydrolases, the most powerful biological catalysts previously known. The hydrolytic rates of phosphate and sulfate monoesters are compared directly, and the misleading impression that the two classes of ester are of similar reactivity is dispelled.